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Originally, they were frontal attacks, where you "yank" the person around and get behind them. Quite common in older jujutsu styles.

It's a fair speculation that in Daito-ryu/aikido, these techniques were integrated as a way to put 'stress' on the body from behind - can one retain a connected body while behind pulled in some direction from behind.

This is a logical supposition because if there were purely a concern for combative readiness from behind, there are a number of more effective ways to take a person downwards or off-center from the rear that no DR or aikido system considers. Therefore, it's very likely that they adapted elements of older combative forms and reworked them as a training device to develop the 'aiki body' from a particular 'angle.'
Ellis Amdur

Ushiro ryote properly begins from a flank attack -- with the fairly obvious implication of an attack on a weapon hand , and both turning nage with the grasped wrist, and moving to exercise control from behind -- which can resolve as either a ryote or kubishime control.

If you put a knife in nage's hand, which uke grabs from a slight flank attack, the proper dynamic in the initiation becomes obvious and natural.

Ryokata is more of front control, but also only makes sense to initiate from the side, with an applied turn of nage to apply a control in kuzushi.

This is a logical supposition because if there were purely a concern for combative readiness from behind, there are a number of more effective ways to take a person downwards or off-center from the rear that no DR or aikido system considers. Therefore, it's very likely that they adapted elements of older combative forms and reworked them as a training device to develop the 'aiki body' from a particular 'angle.'
Ellis Amdur

Oisin - "by what would they be," do you mean what kind of techniques? To enumerate a few from the Araki-ryu (other ryu approached this differently, so Takenouchi-ryu, Shosho-ryu, or Tsutsumi Hozan-ryu might have different takes of this):
1. Grab them by the hair, slam them backwards to concuss and stomp on their head (the name of this technique is "the unification of the topknot and the throat")
2. Simultaneously to grabbing around the neck, stomp the back of the knee and lever your shoulder forward into the back of the neck to hopefully break their neck, but at minimum, putting them in a perfect position to continue into a tracheal strangle (called "falcon stoop")
3. The grab entry, followed by slipping behind (like 'aiki' arts), but continuing with a stab with a short weapon. (called "the truth of grappling")
4. A simple tackle from behind, followed by a pin and a leveraged neck/spine break (called "bringing back to life," for reasons I do not know)

There are more. . . but that's a start. That's what was considered and trained originally.

Oisin - "by what would they be," do you mean what kind of techniques? To enumerate a few from the Araki-ryu (other ryu approached this differently, so Takenouchi-ryu, Shosho-ryu, or Tsutsumi Hozan-ryu might have different takes of this):
1. Grab them by the hair, slam them backwards to concuss and stomp on their head (the name of this technique is "the unification of the topknot and the throat")
2. Simultaneously to grabbing around the neck, stomp the back of the knee and lever your shoulder forward into the back of the neck to hopefully break their neck, but at minimum, putting them in a perfect position to continue into a tracheal strangle (called "falcon stoop")
3. The grab entry, followed by slipping behind (like 'aiki' arts), but continuing with a stab with a short weapon. (called "the truth of grappling")
4. A simple tackle from behind, followed by a pin and a leveraged neck/spine break (called "bringing back to life," for reasons I do not know)

There are more. . . but that's a start. That's what was considered and trained originally.

Ellis Amdur

The grab-rear entry doesn't really apply to daito ryu as I trained anyway. The corpus of ushiro techniques start from rear attacks and work from there. The techniques aren't as apparently violent as the ones you mentioned, but there are a number of rear strangulation, stabbing, kicking, pinning and locking techniques.

I think the origin of rear attacks in aikido is related to a balanced curriculum, with the emphasis on creating a rear-oriented attack for practice. As for ryokata, ryote and the other multiple grab options, you are also talking about a generalized attack attack to managed. I feel the attacks are designed to create a sterile canvas to work on, not necessarily a "practical" attack. Budo Renshu has several photos as well as a selection that speaks specifically about training to defend all angles of attack, even the rear.

Whether those earlier attacks had origin as more "practical", I get the impression for Budo Renshu that we should use [generic] attacks as tools to train the aiki body.